The main sources of conjugated linoleic acid (CLA) in human diet can be milk fat and meat of the ruminants. The term of CLA defines group positional and geometric isomers of linoleic acid (C 18:2), in witch the double bounds are conjugated. Increasing interesting of CLA result from theirs propriety: inhibition (in vitro and in vivo) of cancers cells development (cis-9, trans-11 CLA), retardation of arteriosclerosis development, diabetes mellitus t.2, reduction of fatty tissue (trans-10, cis-12 CLA), improvement of bones mineralisation, immunomodulating effect and bacteriostatic and antyoxydative effect. In rumen, polyunsaturated fatty acids (linoleic and linolenic), which are provided with the fodder, devolved biohydrogenation process by Butyrivibrio fibrosolvents bacterium and others, to stearic acid. In these transformations cis-9, trans-11 CLA isomer and vaccenic acid (VA, trans-11 C18:1) posed the intermediate products. However, 64-78% of cis-9, trans-11 CLA present in milk fat is synthesized in mammary gland from VA by Δ⁹-desaturase (stearoyl-CoA desaturase). The average content of CLA in milk of cows is 4,5-5,5 mg/g of fat. The cis-9, trans-11 isomer pose about 75-90 % (to 95%) of total CLA in milk fat. Content of CLA in milk is determined in the highest extend by nutritional factors, however, there are some others factors as race, age or lactation stadium. There was a growth of CLA in milk fat after use of oils or bruised grains, (in this the extruded) and calcium salts of these oils in cows feeding. The clear modification of quantity and composition of fatty acids in milk fat is observed also after use of fish oil. Fish oil caused the increase of protozoan number in rumen, and supplied EPA and DHA inhibit the reduction of trans-C18:1 to 18:0 during biohydrogenation of PUFA, what leads to the trans-C18:1 accumulation. In the rumen contents and milk the concentration of trans-11 C 18:1 is also grow up. It caused the endogenic synthesis of cis-11, trans-12 CLA growth in mammary gland as an effect of this. The fish oil supplementation of cows diet caused growth of cis-11, trans-12 CLA and VA in milk fat as much as over 200 to 400%. The most effective method is using of fish oil and oils with large content of linoleic acid at the same time. Simultaneously growth of fatty acids of n-3 type (EPA and DHA) was observed. Butter and cheese obtained from milk, which contain higher quantities of CLA and PUFA are very soft, but the gustatory values not differ than commercial products. Undeniable trumps of these products is higher participation of CLA, in this cis-9, trans-11 CLA and n-3 acids, with lower participation of saturated acids.

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